The cooperation between transmission points is an important means to mitigate inter-cell-interference in cellular systems, and it is being intensively discussed in the fourth generation of wireless communication system standardizations. One transmission method of transmission point cooperation is the joint transmission, namely a User Equipment (UE) would receive data signal from multiple transmission points.
FIG. 1 is a diagram showing the operation for joint transmission between two transmission points.
As shown in FIG. 1, the UE #A receives signals from the node (transmission point) #1 and node #2 at the same time, and the UE #B also receives signals from the node #1 and node #2 at the same time. The node #1 and node #2 share data and schedule information through backhaul.
To support the joint transmission, the UE needs to measure the channel from each transmission point, and then report those channels (i.e., Pre-coding Matrix Indices (PMI)/Channel Quality Indicator (CQI)/Ranking Indicator (RI) corresponding to each transmission point). Moreover, to ensure the signals from each transmission point are combined coherently at the UE (but not cancel each other), the UE also reports a phase offset (co-phasing factors) between two PMIs. The transmission point can apply the phase offset on the transmitted data signals to strengthen the signal power received at the UE. In general, the phase offset is quantized before being sent back to the transmission point, for example they are quantized to [1 −1] or [1 j −1 −j].
Moreover, in certain cellular environments, such as interference-limited environments, the UE may receive signals from more than two transmission points, e.g., three transmission points.
FIG. 2A and FIG. 2B are diagrams showing the reporting of multiple phase offsets, in which FIG. 2A shows a situation of homogeneous deployment, and FIG. 2B shows a situation of heterogeneous deployment.
As shown in FIG. 2A, a UE receives signals from three transmission points (cells). In this case, two phase offsets are needed, one of which corresponds to the phase offset φ1 between the serving cell and the neighbor cell 1, and the other of which corresponds to the phase offset φ2 between the serving cell and the neighbor cell 2.
When the UE reports multiple phase offsets, the signal combining gain from multiple transmission points may be cancelled due to the quantization. For example, two phase offsets with similar values may be quantized to completely different values.
FIG. 3 shows an instance that two phase offsets with similar values are quantized to completely different values.
As shown in FIG. 3, both the phase offsets φ1 and φ2 are close to π/2, they have the similar values, and the corresponding signal combining gain should be high. However, if the quantization is [1 −1], for example φ1 is quantized to “1”, and φ2 is quantized to “−1”, the signal combining gain from the multiple transmission points are zero since φ1 and φ2 are quantized to opposite values “1” and “−1”.
FIG. 4 shows an instance that two phase offsets with similar values are quantized to completely different values.
As shown in FIG. 4, the phase offsets φ1 and φ2 are close to phase zero, they have the similar values, and the corresponding signal combining gain should be high. However, if the quantization is [j −j], for example φ1 is quantized to “j”, and φ2 is quantized to “−j”, the signal combining gain from the multiple transmission points are zero since φ1 and φ2 are quantized to opposite values “j” and “−j”.
When the transmission point applies the phase offsets in downlink transmission, the signals from the two neighbor cells may cancel each other due to such quantization.